Stewart, J. P.

The L’Aquila earthquake occurred on April 6 2009 at 03:32:39 local time. The earthquake (Mw=6.3) was located in the central Italy region of Abruzzo. Much of the damage occurred in the capital city of L’Aquila, a city of approximate population 73000, although many small villages in the surrounding region of the middle Aterno river valley were also significantly damaged. In the weeks following the earthquake, the Geo-Engineering Extreme Events Reconnaissance (GEER) international team, comprised of members from different European countries and the U.S., was assembled to provide post-earthquake field reconnaissance. The GEER team focused on the geological, seismological, and geotechnical engineering aspects of the event. We describe the principal seismological findings related to this earthquake including moment tensors of the main shock and two triggered events, the aftershock pattern and its variation with time, tectonic deformations associated with the main shock, surface fault rupture, and the inferred fault rupture plane. We describe damage patterns on a village-to-village scale and on a more local scale within the city of L’Aquila. In many cases the damage patterns imply site effects, as neighbouring villages on rock and soil had significantly different damage intensities (damage more pronounced on softer sediments). The April 6 mainshock was the best-recorded event to date in Italy. We present metadata related to the recording sites and then present preliminary comparisons of the data to GMPEs. Those comparisons support the notion of faster distance attenuation in Italy relative to the average for active regions as reflected in NGA GMPEs. Several incidents of ground failure are then discussed, including a number of rockfalls and minor landslides. Perhaps the most significant incidents of ground failure occurred at Lake Sinizzo, for which we describe a number of slumps and spreads around the lake perimeter. This is documented using field observations as well as LIDAR and bathymetric data.

The L’Aquila earthquake occurred on April 6 2009 at 03:32:39 local time. The earthquake was located in the central Italy region of Abruzzo. Much of the damage occurred in the capital city of L’Aquila, a city of approximate population 73000, although many small villages in the surrounding regions were significantly damaged including Paganica, Castelnuovo, and Onna. Collapsed and damaged structures in L’Aquila included both older masonry buildings and relatively modern reinforced concrete structures. At the time of this writing, 307 people are known to have died from the earthquake, most in collapsed structures, making this the deadliest earthquake to strike Italy since the 1980 Irpinia earthquake. A number of reconnaissance teams were mobilized to the affected region in the weeks following the earthquake. The national institute of geophysics and volcanology (Istituto Nazionale di Geofisica e Vulcanologia, INGV) mobilized a team of geologists (EMERGEO Working Group) to look for evidence of surface rupture and other effects; some of their findings are discussed in this report. The GEER team was assembled to investigate geological, seismological, and geotechnical engineering aspects of the event. The international GEER team is comprised of members from Italy, Austria, Switzerland, Greece, and the United states. Team members were selected to provide needed expertise in geology, engineering geology, GIS applications, earthquake ground motions, and geotechnical earthquake engineering. The team includes individuals highly experienced in post-earthquake reconnaissance and relatively young professionals investigating their first earthquake. The GEER team did not focus on structural engineering or lifeline aspects of the event, which were investigated by an EERI team. The GEER and EERI activities were closely coordinated to optimize resources in the documentation of the valuable, perishable data associated with the earthquake effects. The GEER team employed a number of innovative technologies to facilitate effective reconnaissance. All teams mobilized for field work had a common GPS unit and laptop with a Google Earth (GE) GIS database activity maintained over the course of the work. The GE database was used to keep track of visited locations, but also contained maps of surface geology, locations of aftershocks, strong motion stations, and other information relevant to investigators in the field. Another valuable use of technology involved LIDAR mapping of a site having significant incidents of ground failure (Lake Sinizzo). This report presents the GEER findings. Following this introduction, Chapter 2 describes the geologic and tectonic setting, moment tensor solutions for the mainshock and several triggered events, analysis of aftershock patterns, and analysis of GPS and InSAR data. Included in Chapter 2 is a preliminary model of the ruptured fault. Chapter 3 describes the ground motions recorded during the mainshock by a digital instrument array. Metadata associated with the recordings is presented, trends in the recorded ground motions are presented, and preliminary comparisons to ground motion prediction equations are made. Chapter 4 presents damage patterns, both within L’Aquila and through comparisons of damage intensities in adjacent villages with similar construction. The results provide valuable insights into possible site effects on ground motion in regions where recordings are not available. Chapter 5 presents our findings on ground failure, defined as permanent ground deformations induced by the earthquake. Observed ground failure included several rockfalls, seismic compression of fill materials, and apparent strength loss of soil materials leading to inward movement of the banks of a lake. Chapter 6 reviews the performance of earth dams and earth retaining structures, both of which generally performed well.

The 2009 L’Aquila earthquake sequence includes the April 6 Mw 6.3 main shock and triggered events on April 7 and 9, each recorded on a digital network having five stations on the hanging wall of the main shock fault. We describe a geometric source model drawing upon inversions by others. We describe record-specific ground motion data processing that includes the incorporation of static displacements of up to 13 cm (downdrop of hanging wall). The resulting database includes 47, 38, and 31 corrected triaxial recordings from the April 6, 7, and 9 events, respectively. We present site conditions for recording stations, including recent surface wave and borehole geophysics. We demonstrate that the high-frequency data are weaker than expected for normal fault earthquakes of these magnitudes and that the data attenuate with distance at rates generally consistent with modified next generation attenuation (NGA) equations for Italy that were available prior to the event.